1. Field of the Invention
The invention of the present disclosure provides a novel adaptive image acquisition-compatible intracavitary brachytherapy (ICBT) applicator with remotely-controlled colpostat shields which may be manipulated to minimize computed-tomography image artifacts or to optimize the dose distribution to the target and normal tissue structures for cancer brachytherapy procedures.
2. Description of Related Art
More than 12,000 new cases of cervical cancer are expected to be diagnosed in the United States in 2003 (American Cancer Society, Cancer Facts and Figures 2003). ICBT is an integral part of the treatment regimen for cervical cancer. It is also used in the treatment of other gynecological malignancies, such as vaginal and endometrial cancer. Combined, these cancers account for about 56,000 new cases in the U.S. each year (American Cancer Society, Cancer Facts and Figures 2003) of which about 20% or 11,200 cases would be treated with ICBT procedures. In addition, worldwide, each year more than 600,000 women develop some form of gynecological cancer, according to the World Health Organization.
Traditionally, many cancers of the cervix are treated with radiation therapy. Between 1996 and 2000, about 84% of these treatments in the U.S. were with low dose rate (LDR) 137Cs sources, with the remainder using high dose rate (HDR) 192Ir (Eifel P, et al. Patterns of Radiotherapy Practice for Patients with Carcinoma of the Cervix (1996-1999): A Patterns-of-Care Study. In proceedings of the 45th Annual ASTRO Meeting; 2003). One manner of delivering such radiation is through an ICBT procedure. In an ICBT procedure, radioactive sources are manually or automatically loaded into applicators placed inside the uterine canal during an operative procedure via a procedure termed afterloading. ICBT may, alternatively or additionally, be administered preoperatively or postoperatively and may be paired with external beam radiotherapy, chemotherapy, or both. The targeted cancerous cells or tissue are typically irradiated through the use of a brachytherapy applicator. Current applicators contain left and right ovoids or colpostats and are made of stainless steel. Several varieties of these applicators also have special fixed tungsten shields designed to reduce complications due to inadvertent irradiation of the rectum, bladder or other surrounding tissue. The current practice for positioning of the shield alignment with the bladder and rectum depends on the patient's anatomy and physician's skill.
Additionally, the size, shape, thickness and positioning of these shields may have a substantial effect on the dose of radiation received by normal tissues proximal to the targeted site, particularly the rectum in the case of cervical cancer, and complication rates have been shown to be directly dependent on the dose received by these organs. The clinical treatment planning systems currently used, however, typically are unable to accurately account for the effects of shields resulting in errors of 30% or more in the predicted dose to critical organs (Mohan R, et al. Int J Radiat Oncol Biol Phys 1985a; 11 (4):861-8.; Mohan R, et al. Int J Radiat Oncol Biol Phys 1985b; 11 (4):823-30; Weeks K J, Med Phys 1998; 25 (12):2288-92; Williamson J F, Int J Radiat Oncol Biol Phys 1990; 19 (1): 167-78). Other studies have shown that dose perturbations resulting from inter-source shielding and applicators are also clinically significant and should be modeled. Fragoso, et al. found that errors as large as 20% could result from not explicitly modeling the steel ovoid applicators and source spacers in LDR treatments (Fragoso M, et al. In Proceedings of the 2003 AAPM Annual Meeting; 2003). Gifford, et al. concluded that explicit modeling of the tandem applicator was also important. Intra-source and inter-source attenuation and the presence of a tip screw were found to have significant effects on the local dose field (Gifford K, et al. In Proceedings of the 2003 AAPM Annual Meeting; 2003).
An integral component in determining the dose distribution to be received by the targeted and non-targeted areas is the positioning of any radiation shielding within the ovoid. ICBT dose distribution planning often involves the use of three dimensional visualization of the targeted areas and surrounding anatomical structures to determine the appropriate position of the implanted applicator in order to maximize a dose distribution of the radiation over the targeted areas. Techniques such as computed-tomography (CT), magnetic resonance (MR), or positron emission tomography (PET) have been employed in the past to generate a three dimensional treatment plan for ICBT procedures. Such techniques are limited by the fact that the shields used in ICBT applicators can interfere with these various methods of planning by distorting images of the implant localization and causing streak artifacts, making a determination of the optimal position of the applicator within the body cavity very difficult to determine.
U.S. Pat. No. 5,562,594 discloses a CT-compatible applicator design (the “Weeks” applicator) that permits CT 3D dosimetry (Weeks K J and Montana G S, Int J Radiat Oncol Biol Phys 1997; 37 (2):455-63). The Weeks ovoid has tungsten-shielded source carriers which are after-loaded post CT image acquisition. The external shape of the Fletcher-Suit-Declos (FSD) minicolpostat tandem and ovoids system appears to have been the basis for the shape of the Weeks applicator. However, the fixed Fletcher-like shields have been removed and replaced with tungsten shields which are manually loaded in conjunction with the 137CS sources.
The Weeks applicator has been used to develop a technique for improved CT based applicator localization (Lerma F A and Williamson J F, Med Phys 2002; 29 (3):325-33). This study demonstrated that it was possible to support 3D dose planning involving detailed 3D Monte Carlo dose calculations, modeling source positions, shielding and inter-applicator shielding accurately. Nevertheless, the Weeks applicator has several disadvantages. For example, the Weeks applicator is not adaptable to remote afterloading (loading the radioactive source into the applicator post-insertion and positioning within the body cavity) thereby increasing the radiation exposure from LDR brachytherapy; and it cannot be used at all for HDR or pulsed dose rate (PDR) applications. In addition, in order to accommodate the afterloading shields, the arms connected to the ovoids are much more bulky than those of a standard FSD applicator. The increased size of the arms makes it more difficult to insert the vaginal packing needed to distance the bladder and rectum from the radiation sources. This added bulk also has a potentially negative impact on the comfort of the patient undergoing treatment.
Another available commercial option is the “Standard CT/MR Applicator” based on a Royal Marsden design from Nucletron Corporation. It is designed with special composite tubing to eliminate distortion on CT or MR images. This applicator is available in different lengths and ovoid diameters to optimize the dose distribution and reduce the mucosal dose. This applicator was not designed for use with any shielding, however, and thus its use results in exposure of the rectum and bladder or other surrounding tissue to unnecessarily high doses of radiation which may lead to clinical complications.
Therefore, a need exists for a brachytherapy applicator that is amenable to radiation source afterloading but still capable of being manipulated to allow for enhanced image acquisition with minimal artifact generation.